Research ArticleGEOPHYSICS

Seismic evidence for megathrust fault-valve behavior during episodic tremor and slip

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Science Advances  22 Jan 2020:
Vol. 6, no. 4, eaay5174
DOI: 10.1126/sciadv.aay5174
  • Fig. 1 The study region in the forearc of the Cascadia subduction zone.

    Station locations considered for RF analysis are shown by triangles (solid triangles mark stations where changes in velocity structure are observed). The yellow square marks the location of the GPS station used to estimate ETS timing. Contours mark the depth of the plate interface inferred from the deformation front, up, and downdip limits of tremor and volcanic arc (19). The gray shaded region delineates the ETS source region interpreted from tremor density (19). The inset shows the tectonic setting of the Cascadia subduction zone.

  • Fig. 2 ETS-related changes in velocity structure beneath station PGC.

    Changes in mean CC values for 15-day time bins before and after ETS are shown for SV (A) and SH (B) data. Changes in mean δvs before and after ETS are shown for the top (C) and bottom (D) boundaries of the LVL. Mean values and estimated standard errors in (A) to (D) are shown by horizontal lines and shaded regions, respectively. Changes in RF data as a function of time bin duration are shown for CC analysis (E) and δvs analysis (F). Bootstrap resampling of RF times relative to ETS are shown by gray lines (500 of 20000 samples), and 95% confidence levels are shown by dashed lines in (E) to (F).

  • Fig. 3 Conceptual model of ETS-related fluctuations in Pf in Cascadia.

    During interslip periods (A), the deep extension of megathrust faults where ETS occurs is characterized by high Pf. During ETS (B), low-permeability seals within the plate interface shear zone are breached, which opens permeable pathways for fluid migration within and across the megathrust, causing a drop in Pf. Low-permeability barriers are re-established, and the cycle repeats. This figure is adapted from Peacock et al. (10).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/4/eaay5174/DC1

    Fig. S1. SV RFs for station PGC.

    Fig. S2. Harmonic components for station PGC.

    Fig. S3. Cross-correlation coefficients between observed and predicted RFs.

    Fig. S4. Example calculation of strike and dip angles at each station location.

    Fig. S5. Estimated S-wave velocity contrast across the top and bottom of the LVL.

    Fig. S6. GPS data used in the determination of ETS event times.

    Fig. S7. Changes in δvs across the top and bottom boundaries of the LVL.

    Table S1. Strike and dip angles of the plate interface at each station location.

    Table S2. Estimated start, end, and midpoint dates of ETS events from 1994 to 2019.

    Table S3. P values from the t tests for the change in CC coefficients and δvs values following ETS events measured at station PGC.

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. SV RFs for station PGC.
    • Fig. S2. Harmonic components for station PGC.
    • Fig. S3. Cross-correlation coefficients between observed and predicted RFs.
    • Fig. S4. Example calculation of strike and dip angles at each station location.
    • Fig. S5. Estimated S-wave velocity contrast across the top and bottom of the LVL.
    • Fig. S6. GPS data used in the determination of ETS event times.
    • Fig. S7. Changes in δvs across the top and bottom boundaries of the LVL.
    • Table S1. Strike and dip angles of the plate interface at each station location.
    • Table S2. Estimated start, end, and midpoint dates of ETS events from 1994 to 2019.
    • Table S3. P values from the t tests for the change in CC coefficients and δvs values following ETS events measured at station PGC.

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